clk: imx8mm: add enet clk

Add enet ref/timer/PHY_REF/root clk which are required to make enet
function well.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>